The invention relates to a strain detector to be attached to the shafts of devices such as engines, motors, or electromagnetic clutches.
FIG. 1 shows an exemplary construction of a conventional strain detector. Reference numeral 1 designates a driven shaft which is a rotating shaft; 2, the axis of rotation of the driven shaft 1; and 3, 4, bearings for rotatably supporting the driven shaft 1.
On the outer peripheral surface of the driven shaft 1 are magnetic layers 5, 6 having a high permeability and being firmly fixed of the driven shaft so as to be spaced apart from each other along the axial direction of the driven shaft. The magnetic layer 5 is arranged so that a plurality of narrow, long strands extend at an angle of +45 degrees with respect to the axis 2, while the magnetic layer 6 is arranged so that a plurality of narrow, long strands extend at an angle of--45 degrees with respect to the axis 2. Around the outer peripheries of the magnetic layers 5, 6 is a cylindrical coil bobbin 7 arranged so as to be concentric with the driven shaft 1. Around the outer peripheral surface of the coil bobbin 7 are detecting coils 8, 9 wound so as to be in opposition to the magnetic layers 5, 6, respectively. The detecting coils 8, 9 are connected to a detection circuit 14. Reference numerals 10, 11 designate magnetic converging layers arranged around the detecting coils 8, 9. These magnetic converging layers are made of a soft magnetic material having a high permeability such as an amorphous alloy or a silicon steel strip. Reference numerals 12, 13 designate metallic yoke layers arranged around the magnetic converging layers 10, 11, respectively. These metallic yoke layers are made of a nonmagnetic and highly conductive material such as copper or aluminum and formed into a cylinder-like profile.
When a torque is applied to the driven shaft 1 from outside, a tensile force is generated at one of the magnetic layers 5, 6 and a compressive force, at the other, thus leaving the shaft 1 under strain. Such a strain then causes the permeability of each magnetic layer 5, 6 to change. The tensile force and the compressive force act to change each permeability in directions which are opposite to each other.
Since each magnetic converging layer 10, 11 is highly permeable and has a small magnetic resistance, the magnetic flux density of each magnetic layer 5, 6 is increased, improving not only the sensitivity but also the resistance to externally-induced noise with suppressed expansion of the magnetic flux. Each metallic yoke layer 12, 13 that is nonmagnetic and highly conductive has a small depth of penetration of magnetic flux due to its skin effect and this better confines the magnetic flux, thus leading to improvements in the sensitivity and noise resistance.
FIG. 2 shows an exemplary configuration of a detection circuit utilized in the above type of strain detector (Japanese Patent Unexamined Publication No. 154128/1990; Date of Publication: on Jun. 13, 1990). In FIG. 2, reference numeral 24 designates an output terminal of an intermediate potential of the detecting coils 8, 9 that are connected in series with each other; 27, an AC drive circuit for applying an AC voltage across the series-connected body of the detecting coils 8, 9; 25a, 25b, differential amplification circuits for detecting a potential difference between the terminals of the detecting coils 8, 9 with the voltage of the intermediate potential output terminal 24 as a reference voltage; 26, an addition circuit for adding an output of the differential amplification circuits 25a, 25b ; 28, a synchronized detection circuit for detecting an output of the addition circuit 26 base on a synchronized detection signal from the AC drive circuit 27; 29, a circuit for smoothing/amplifying/AC-DC converting a pulsating output signal of the synchronized detection circuit 28; and 30, an output terminal.
FIG. 3 shows a specific configuration of the AC drive circuit 27. In FIG. 3, reference characters Q.sub.1 to Q.sub.4 designate transistors; V.sub.0 to V.sub.2, an intermediate potential and a voltage across the series-connected body of the detecting coils 8, 9; and S.sub.1 to S.sub.4, timing signals for operating the transistors Q.sub.1 to Q.sub.4. FIG. 4 is a timing chart of the operation of the AC drive circuit 27. When S.sub.1, S.sub.2 are low and S.sub.3, S.sub.4 are high, Q.sub.1, Q.sub.4 are on, while Q.sub.2, Q.sub.3 are off. As a result, V.sub.1 =V.sub.cc and V.sub.2 =0, which causes a positive current to flow through the detecting coils 8, 9. Conversely, when S.sub.1, S.sub.2 are high and S.sub.3, S.sub.4 are low, Q.sub.1, Q.sub.4 are off, while Q.sub.2, Q.sub.3 are on. As a result, a negative current flows through the detecting coils 8, 9.
FIG. 5 shows an exemplary configuration of the synchronized detection circuit 28. In FIG. 5, Q.sub.5 designates a transistor; 42, an input terminal for receiving an output of the addition circuit 26; 43, an output terminal thereof; and 44, a resistor. FIG. 6 is a timing chart showing the operation of the synchronized detection circuit 28, of which part (a) shows a synchronized detection signal, which is received by the base of the transistor Q.sub.5 from the AC drive circuit 27 and is out-of-phase with an AC drive signal applied from the AC drive circuit 27 by a predetermined degree; part (b) shows the waveform of an output from the addition circuit 26; part (c) shows the on-and-off operation of the transistor Q.sub.5 ; and part (d) shows the waveform of an output from the output terminal 43. When the synchronized detection signal is high, the transistor Q.sub.5 is turned on and no output signal of the addition circuit 26 is outputted from the output terminal 43. When the synchronized detection signal is low, the transistor Q.sub.5 is turned off and an output of the addition circuit 26 is directly applied from the output terminal 43. The noise component in the output of the addition circuit 26 is thus removed by the synchronized detection circuit 28.
An operation of the circuits thus configured will be described next. The AC drive circuit 27 outputs an AC voltage and an AC current as shown in FIG. 4, and the output is applied across the series-connected body of the detecting coils 8, 9. Each detecting coil 8, 9 detects a change in the permeability of each magnetic layer 5, 6 as a change in its self-inductance. The output across each of the detecting coils 8, 9 is an induced voltage corresponding to the change in the self-inductance, and is applied to each of the differential amplification circuits 25a, 25b with the intermediate potential as a reference voltage, so that the difference between these applied induced voltages is amplified. The outputs of the differential amplification circuits 25a, 25b are added at the addition circuit 26 and detected thereafter at the synchronized detection circuit 28 by a synchronized detection signal that has been synchronized with one of the drive timings, either positive or negative, applied from the AC drive circuit 27 to remove the noise component. A signal pulsated by the synchronized detection circuit 28 is subjected to a smoothing and amplifying process by the AC-DC conversion circuit 29 and applied from the output terminal 30 as a strain detection signal.
Since the excitation and driving of the detecting coils 8, 9 and the detection of a change in the permeability of each magnetic layer 5, 6 are performed at different circuits in the aforesaid detection circuit, it is easy to make adjustments in circuit characteristics. In addition, the AC drive circuit 27 allows the drive current flow to be large in amplitude, which contributes to expanding the operating magnetic field region. As a result, the influence of an externally acting magnetic field can be reduced.
However, such a detection circuit effects its synchronized detection at only one timing, forward-driving or reverse-driving, of the AC drive circuit 27, and this disadvantageously causes the output of the synchronized detection circuit 28 to incorporate error components.
FIG. 7 shows a detection circuit in which the above problem has been overcome (Japanese Patent Unexamined Publication No. 271229/1990; Date of Publication: Nov. 6, 1990) In FIG. 7, reference numerals 28a, 28b designate synchronized detection circuits for detecting an output of the addition circuit 26 in synchronism with a synchronized detection signal. The synchronized detection circuit 28a detects the output with a first synchronized detection signal which is in synchronism with a positive drive timing of the AC drive circuit 27, while the synchronized detection circuit 28b detects the output with a second synchronized detection signal which is in synchronism with a negative timing of the AC drive circuit 27. The circuit configuration of each synchronized detection circuit 28a, 28b is the same as that of FIG. 5. Reference numeral 31 designates a differential amplification circuit which amplifies the difference between the outputs of the synchronized detection circuits 28a, 28b.
An operation of the detection circuits shown in FIG. 7 will be described with reference to a timing chart shown in FIG. 8. As shown by part (a) of FIG. 8, the AC drive circuit 27 outputs a drive voltage at drive timings, both positive and negative, while as shown by part (b) thereof, the addition circuit 26 outputs a strain component. The first synchronized detection signal is applied from the AC drive circuit 27 in synchronism with a positive drive timing as shown by part (c) of FIG. 8, and the second synchronized detection signal is applied from the AC drive circuit 27 in synchronism with a negative drive timing as shown by part (d) thereof. The synchronized detection circuits 28a, 28b output detection outputs shown by parts (e), (f) of FIG. 8 in accordance with the first and second synchronized detection signals. The differential amplification circuit 31 produces an output shown by part (g) of FIG. 8 while amplifying the difference between the applied outputs of the synchronized detection circuits 28a, 28b. The output from the differential amplification circuit 31 is then smoothed and amplified by the AC-DC conversion circuit 29 and applied from the output terminal 30 as a strain detection signal.
In the detection circuits shown in FIG. 7, the strain-based outputs are detected at both positive and negative drive timings of the AC drive circuit 27, and the difference between the detected outputs is amplified, thereby canceling the error components out. As a result, not only can the strain detection accuracy be improved, but also the influence of an externally applied magnetic field can be canceled out.
The aforesaid strain detector is attached to a device such as an engine, a motor, or an electromagnetic clutch to control its transmitted torque or the like. However, the engine or the like generates heat and the generated heat is transmitted to the detector through the driven shaft 1 that is connected thereto, providing a temperature gradient with the driven shaft 1. This further generates a difference in the temperature between the magnetic layers 5, 6, leading to a difference in permeability. As a result, the detection values incorporate heat-induced errors.
Further, each of the aforesaid detection circuits produces only one output, and this has hampered detection of abnormality caused by, e.g., heat. The detecting coils 8, 9 are connected in series with each other to divide the power voltage by their inductances and internal resistances. As a result, the output of each of the detecting coils 8, 9 includes the inductance of the other no matter how it is post-processed. Inability of measuring the behaviors of the detecting coils independently of each other does not allow a change in the inductance due to application of an external force to be separated from a change in the inductance due abnormalities of the magnetic layers 5, 6 or the like. As a result, abnormalities in the magnetic layers 5, 6 and the detection circuits cannot be detected, which is another disadvantage.